The increasing clinical use of propranolol here and abroad as an antihypertensive, has not been paralleled by a growth of knowledge concerning the mechanism for this action. The well known initial effect on cardiac output may, in fact, be accompanied by an increase in total peripheral resistance and by no change in blood pressure. Total peripheral resistance and blood pressure only began to fall after 4-6 weeks of relatively high dose regimens. Changes in renal metabolism occur not at all or in variable directions and often after pressure has been lowered. Speculative but untested attention has been given to the carotid sinus. Except for evidence that propranolol reduces carotid chemoreceptor activity, there have been no studies on the effect of propranolol on carotid depressor nerve activity (CSN). We propose to study in cats the effect of propranolol on CSN activity in the perfused carotid sinus and chronically in cats by challenging the sinus nerve with differing levels of perfused pressure and levels of propranolol, its metabolites and cogners.